1. Field of the Invention
This invention relates to a wear warning device for friction pads for use in a disc brake in a vehicle, and more particularly to a wear warning device to warn the driver to get the brake service by means of the pulsation transmitted to the brake pedal, i.e., to warn the driver on the fact that the friction pad reaches a wear limit.
2. Description of the Prior Art
One of the prior art friction-pad wear warning devices is shown in FIG. 1. As disclosed in U.S. Pat. No. 3,918,553, the warning device includes; radially protruding portions 14' which are flush with the inner edge portion of the friction surface 12' of a brake disc 10' and extend radially inwards of the brake disc 10'; and a pressure bearing portion 22' extending from a friction pad and having a surface flush with a wear-limit surface 20' of the friction pad. Accordingly, when the friction pad 18' is worn down beyond a wear limit, then the pressure bearing portion 22' abuts the protruding portions 14' on the brake disc 10'.
In the disc brake equipped with the aforesaid warning device, when a brake pedal is pushed down, then a force of a brake pedal is delivered via a hydraulic system to a piston in a disc brake, so that a piston forces a pad unit against a brake disc for the braking action. In case a friction pad is not worn down to a wear limit upon braking, then the pressure bearing portion 22' does not contact the protruding portions 14', and hence the warning device does not give off a warning. However, in case a friction pad reaches a wear limit, then the surface 20' of the friction pad becomes in alignment with the pressure bearing portion 22'. As a result, the protruding portions 14' on the disc abut the pressure bearing portion 22'. As a result, the pressure bearing portion 22' intermittently receives reactions from the protruding portions 14' positioned in non-continuous or spaced relation along the circumference of a brake disc, and then vibrations caused by the reactions are delivered from the piston, through a hydraulic circuit adapted to operate the piston, to a brake pedal in the form of pulsation. The pulsation of a brake pedal upon braking action signifies that a friction pad reaches a wear limit, and thus the driver may sense pulsation as a warning.
Meanwhile, the protruding portions 14' disposed along the circumference of a brake disc in the prior art warning device of this type are formed with edges. Accordingly, when the protruding portion 14' abuts the pressure bearing portion 20', then the edge of the protruding portion 14' damage the pressure bearing portion 22'. This would cause the pressure bearing portion to come off a friction pad, due to the contact of the protruding portions 14' with the pressure bearing portion 22' only several times. Due to this fact, the prior art warning device can give a warning only for a quite short period of time.